The present invention relates to rigid polyurethane foams, particularly rigid polyurethane foams that are useful as reinforcement materials, such as in the auto industry.
Polyurethane foams have been used in the auto and other industries for a number of purposes. For example, rigid foams have been used for structural reinforcement, preventing corrosion and damping sound and vibration. These foams are typically formed by applying a reactive foam formulation to a part and allowing the formulation to foam in place. The part is often already assembled onto a vehicle when the foam is applied. This means that the foam formulation must be easy to mix and dispense, must cure rapidly before it runs off the part, and preferably initiates curing at moderate temperatures. To minimize worker chemical exposure, the formulation is preferably is low in volatile organic compounds, especially volatile isocyanates and amines. The individual components are preferably storage-stable at room temperature for an extended period.
One foaming system for these applications is based on the prepolymer described by Rizk et al. in U.S. Pat. No. 5,817,860. The prepolymer described in that patent is made by reacting an isocyanate with a monofunctional alcohol and a polyol. Foams are made from this prepolymer by reacting it with water. While good quality rigid foam can be made using this approach, it has several drawbacks. First, because the prepolymer is cured with a water stream, the volume ratio of the reactants (prepolymer and water) is often quite high, such as 15:1 or more. Much of the commercially available dispensing equipment cannot handle such high component ratios. Second, in order to get a sufficiently fast reaction with this system, it is often necessary to preheat the components to temperatures of 80° C. or more. This increases energy costs, exposes workers to high temperature reactants and reduces the viscosity of the system, thereby promoting run-off.
Approaches for improving or modifying the system of U.S. Pat. No. 5,817,860 are described in WO 02/079340A1, WO 03/037948A1 and U.S. Pat. Nos. 6,541,534 and 6,423,755. These approaches include the use of special acrylate- or methacrylate-functional materials to allow for lower curing temperatures, and the use of hollow microspheres to balance density and compressive strength.
Another approach is to prepare the foam from an epoxy resin. Epoxy resins have the advantage of superior thermal properties. However, epoxy resins suffer from several problems, including a slow cure, a large exotherm and brittleness. As a result, epoxy foams are ill-suited for many applications, particularly vehicular reinforcement applications.
Because a fast cure is needed in many of these applications, a continuing desire is to further increase the rate at which the foam system cures. Faster cure allows lower viscosity components to be used, as the fast cure causes the foam formulation to set before it has time to run off when applied to a substrate. Lower viscosity components are often more easily mixed, and require less energy and frequently less robust equipment to dispense them. However, the achievement of a faster cure must not come at the expense of a desirable foam density and necessary physical properties.
Very rapid cures can be obtained by increasing catalyst levels and/or using highly reactive components. The problem with these approaches is that they disorder the sequencing of the blowing and gelling reactions. This causes the foam to have a higher-than-expected density, poor physical properties, and sometimes an incomplete cure.
It would therefore be desirable to provide a rigid polyurethane system that cures very rapidly to a good quality, rigid form, particularly one that can be applied at lower volume ratios and can be applied at moderate operating temperatures.